During a conventional radiographic analysis, a radio opaque liquid is ingested, injected or otherwise entered into the patient and the progress of the liquid through the patient is monitored by using a low-energy x-ray on a fluoroscopic screen. Often, during this procedure, it is advantageous for the doctor to be able to increase the intensity of the x-ray beam sufficiently to expose a photographic film in order to make a permanent image of the status of the radio opaque liquid at a particular point in its progress.
For example, during a conventional radiographic gastrointestinal analysis, the patient ingests radio opaque liquid which conventionally contains barium. When the patient ingests the radio opaque liquid, the doctor turns on the x-ray generating tube at a low vision level and positions the patient between the x-ray tube and a fluoroscopic screen. The doctor can analyze the patient's gastrointestinal tract while the barium flows through it. When the doctor sees a part of the procedure he wants to record, he typically replaces the fluoroscopic screen with a photographic plate and increases the x-ray to a level intense enough to expose that plate.
Since the liquid is continuously moving, and the image therefore continuously changing, in order for the doctor to get the exact image he desires the x-ray tube must be switchable from the low level fluoroscopic x-ray emission to the high level emission for photographic exposure within a very short time.
Conventionally, the switch from the low level fluoroscopic emission level to the high level photographic exposure level has been made by changing the current through the tungsten filament in the x-ray tube. However, in a conventional x-ray tube the x-rays are produced by generating electrons by thermionic emission from a tungsten filament. The electrons are then accelerated to an anode (which may be rotating for wear averaging purposes) to generate the x-rays. The emission intensity of the tube is controlled by the filament current which in turn controls the number of electrons available to be accelerated to the anode. In order to generate a sufficient number of electrons for either fluoroscopy or photographic exposure, a large current must be passed through the filament. Typical filament currents are approximately three-tenths to five amperes. This means that the tungsten filament must be made of large-diameter tungsten wire. Accordingly, there is a significant thermal time lag associated with changing the emission level of the tungsten filament by changing the current passing through the wire. A typical time lag is approximately one-half second between changing current and the associated change in emission level.
A tube can be made to switch faster by switching the accelerating voltage at the x-ray tube anode with high voltage switching devices such as tetrodes or silicon controlled rectifiers, than by changing the emission current. The filament in such tubes is maintained at a constant high temperature. However, this approach also has drawbacks. In particular, the high voltage tetrodes and silicon controlled rectifiers which are used to switch the anode voltge to the x-ray tube are often expensive and easily damaged. In addition, the x-ray tube generally continues to emit x-rays after the signal has been removed. This occurs because the silicon controlled rectifiers or high voltage switches are connected to the anode by means of high voltage cables. These cables have some significant capacitance which must be discharged fully before the tube ceases to emit x-rays. Thus, after the high voltage switch has turned off the high voltage, the tube continues to emit x-rays as the cables discharge. These unwanted x-rays are often at a much lower energy due to the fallen voltage as the cable discharges and thus radiate the patient without adding to the photographic image as the low voltage x-rays are often of too low energy to penetrate the patient. Thus the patient receives unwanted radiation while no improvement in image quality is achieved.
Further prior art attempts to solve this problem have focused on grid control systems. More particularly, in a conventional x-ray tube, the cathode consists of a metal cup containing two or more filaments. Filaments are different sizes to produce different sized focal spots on the anode. Each of the filaments is located at the bottom of a cup-shaped 5 depression which focuses the negatively charged electron beam on a positive anode. Without the focusing action of the focus cup, the mutual repulsion of the electrons would spread the beam, resulting in an unacceptably large focal spot. In most grid control systems, a negative bias potential on the order of four kilovolts is applied to the focus cup. This potential is large enough to repel the electrons back to the filament, resulting in a cutoff of the x-ray tube emission. However, in a conventional x-ray tube, the filaments are all mounted in the same physical metal cup structure and thus, in order to change emission level, the filaments must be separately turned off, resulting in the same time lag problem as with a single filament tube.